1. Field of the Invention
The present invention relates to a method of preventing deterioration of film quality of a transparent conductive film, a semiconductor device and the method of manufacturing a semiconductor device.
2. Description of the Related Art
A portable information processor is indispensable for performing electronic data processing (EDP) on an individual level, and a liquid crystal display unit is used as man-machine interface required for realizing EDP. It is desired for an image display unit to realize display of high density and high color rendering capable of performing more advanced information processing of image processing or the like at a low cost.
An active matrix type liquid crystal display unit has a construction in which switching elements are connected to all of a plurality of display units called picture elements, thus making high grade display possible. A semiconductor element such as a diode and a transistor is used as the switching element.
The characteristic of the switching element is required to be excellent since it is one of the primary factors for determining the number of gradation and the display capacity of the liquid crystal display unit. Further, since these switching elements are manufactured through a complicated process accompanied by fine processing, the process thereof controls simplification of the process for the whole liquid crystal display unit.
As a means for simplifying the manufacturing process of the switching element, it is being considered to form a new pattern on the pattern already formed on a substrate in the self align manner. There is a technique for growing a thin film selectively only on a conductive pattern as one of processes adopting such a line of thinking of self alignment.
Since it is possible in selective growth to form a layer selectively only on a thin film applied with patterning on a substrate, a photolithography process becomes unnecessary. Further, since it is possible to grow still another thin film continuously without breaking vacuum after the selective forming process, contamination on interlayer interfaces is little, and the switching element characteristic is improved.
Now, in a plurality of selective growth techniques that have been heretofore developed, a chemical vapor deposition (CVD) method is utilized so as to achieve selective growth by the difference in chemical property of layer surfaces.
The CVD method is used frequently as a growth method of a semiconductor or a metal, but the growth rate is not controlled normally under such a condition that reflects the chemical disposition of the layer surface. Therefore, selective deposition of a film does not appear normally. However, it has become possible to perform selective growth by reducing a deposition rate by diluting growth gas with another gas or by having the chemical disposition of the layer surface reflected by bringing etching and deposition close to an equilibrium state.
As a technique of selective growth of silicon, there is an epitaxial lateral overgrowth (ELO) method disclosed in Japanese Patent Provisional Publication No. SHO58-120595 for instance. This is a method of expanding a margin of a selective growth of silicon by repeating deposition and etching of layers alternately for a substrate having a region where silicon is grown and a region where silicon is difficult to be grown.
It has been disclosed in U.S. Pat. No. 5,242,530 that this technique is also applicable to a plasma CVD method widely used in the manufacture of a solar cell and an active matrix type liquid crystal display unit.
The feature of these techniques is to repeat etching and deposition alternately by utilizing a slight difference in a silicon growth rate produced by the difference between materials forming the layers.
However, as the chemical state of the surface of the layer depends on the layer constituting material, growth had to be performed under a condition of a very narrow margin in order to achieve selective silicon growth between materials having closely resembling chemical dispositions.
Accordingly, when the film forming rate is increased in order to shorten the process period of time, it gets out of the condition of selective growth. Thus, such a problem that the film forming process has to be performed for a long period of time by slowing down the film forming rate for performing selective growth has been caused.
It is inevitable to increase the percentage of etching occupying in the total process period of time for surely removing the silicon film deposited in disused parts in order to make selective growth uniform in an active matrix having a large area. As a result, the effective film deposition period of time is reduced and the total selective growth process period of time gets longer, thus producing a main cause of deteriorating the productivity.
Further, it depends on the material constituting the surface of the layer where the film is grown whether silicon is grown or not. Furthermore, the state of the layer surface is also changed by contamination and natural oxidation. Therefore, when the technique of selective growth is examined practically from a viewpoint of a manufacturing process, it has more or less lacked practicability.
For example, ITO shows a disposition of a electrically degenerated semiconductor and functions as an electrode, but resembles in a chemical disposition to silicon oxide that is an oxide such as quartz and glass constituting the substrate since ITO is an oxide, and there is little difference in the processes that silicon is grown on ITO and the substrate. Further, the reducing gas used when silicon is grown selectively on the surface of the transparent conductive film deteriorates the surface of the transparent conductive film.
Accordingly, it is difficult to perform selective growth on either one of two films having resembling chemical dispositions. Practically, according to the examination of the present inventor, it has been found that the reproducibility of selective growth is poor, and such results are brought about that non-selective deposition of silicon films is produced frequently and the silicon film is deposited on the quartz substrate only.